Apparatus of making collimators for precise alignment of optical paths without need for adjusting positions or angles to compensate for offsets or deviations during optical device assembly

ABSTRACT

An apparatus is provided for making a collimator for precise alignment of an output optical path emanating from the collimator to obviate the need for adjusting the collimator during assembly to an optical device to compensate for a translational offset and an angular deviation. In an embodiment, the apparatus is capable of determining the translational offset and the angular deviation, and removing a portion of an outer sleeve of the collimator according to the offset and the deviation to form an outer cylindrical surface that is centered about an axis which coincides with the optical path emanating from the collimator, to allow the collimator to be readily fitted to an optical device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical collimators, and moreparticularly, to an apparatus of making collimators for precisealignment of optical paths.

2. Background Art

Optical collimators have been widely used in optical fibercommunications networks, systems and devices to collimate lighttransmitted by optical fibers, in order to form substantially parallellight beams in free space in various types of optical devices,especially non-integrated optical devices, including, for example,optical switches, isolators, attenuators, beam splitters and beamcombiners. Collimators perform an important function of preventingexcessive insertion loss due to dispersion of light beams in free spacein these optical devices.

FIG. 1 shows a side sectional view of a conventional collimator with agraded index (GRIN) lens for collimating a light beam transmitted by anoptical fiber. In FIG. 1, a section of optical fiber 2 has a terminal 4connected to a capillary 6, which has an index of refraction n₁. Thecapillary 6 typically has a cylindrical shape with a center axis 8. In aconventional collimator with a GRIN lens 12, the capillary 6 typicallyhas an end surface 10 which is slanted slightly off the normal to thecenter axis 8, in order to prevent total reflection of an incoming lightbeam received from the optical fiber 2 back to the optical fiber. TheGRIN lens 12 also typically has a cylindrical shape centered about theaxis 8 and an end surface 14 opposite the end surface 10 of thecapillary 6.

In a conventional collimator, the end surface 14 of the GRIN lens 12 isalso slanted slightly off the normal to the center axis 8. A gap 16 istypically provided between the end surface 10 of the capillary 6 and theend surface 14 of the GRIN lens 12. Both of these end surfaces may beslanted at an angle of about 8° off the normal to the center axis 8, forexample, and are made to be substantially parallel to each other. In aconventional collimator, the gap 16 is typically filled with a gas suchas air, which has an index of refraction n₂, while the GRIN lens 12 hasan index of refraction n₃. In a conventional collimator equipped with aGRIN lens, the index of refraction n₃ of the GRIN lens is typicallydifferent from the index of refraction n₁ of the capillary 6 becausethey are made of different materials. Because of the differences betweenthe indices of refraction n₁ and n₂ at the end surface 10 of thecapillary 6 and between the indices of refraction n₂ and n₃ at the endsurface 14 of the GRIN lens 12, an incoming light beam that enters thecapillary 6 along the center axis 8 typically deviates from the centeraxis 8 at an angular deviation α with respect to the center axis whenthe light beam exits the GRIN lens 12.

Both the capillary 6 and the GRIN lens 12 are enclosed by a cylindricalmetal sleeve 18, which may be made of gold plated stainless steel, forexample, with a inner cylindrical surface 20 and an outer cylindricalsurface 22 centered about the center axis 8. One or more concentriccylindrical layers of protective materials may be provided between theinner surface 20 of the metal sleeve 18 and side walls of the capillary6 and the GRIN lens 12, depending upon the construction of thecollimator. Because of process variations in the manufacturing of aconventional collimator such as the one shown in FIG. 1, slightvariations in the angles of the slanted end surfaces 10 and 14 of thecapillary 6 and the GRIN lens 12 may result in unpredictability of theangular deviation α of the output light beam 24 with respect to thecenter axis 8 of the collimator.

Furthermore, because the cylindrical collimator may be rotatedunpredictably when it is assembled to an optical device, the directionof the output light beam 24 emanating from the collimator is even moreunpredictable. In addition, the incoming light beam that enters thecapillary 6 of the collimator from the optical fiber 2 may not beperfectly aligned with the center axis 8 of the collimator, therebycausing a translational offset Δ in addition to the angular deviation αwith respect to the center axis. Other process variations such astolerance of GRIN lens specifications may also contribute to theunpredictability of the direction of the output light beam emanatingfrom the collimator.

When conventional collimators such as the one shown in FIG. 1 anddescribed above are assembled to an optical device in which at leastsome of the light beams need to travel in free space between thecollimators, alignment of light beams between different collimators canbe very difficult and time-consuming. Translational offset and angulardeviation of light beams emanating from collimators usually exist andare usually unpredictable regardless of the types of lenses used, suchas conventional GRIN lenses, ball lenses or C lenses, even if they aremanufactured with tight specifications. An output light beam emanatingfrom a conventional collimator typically has a very small spot size witha diameter as little as 200 μm, for example. Therefore, even a slightoffset or deviation may cause misalignment of optical paths betweencollimators in an optical device.

FIG. 2 illustrates a simplified sectional view of a typicalnon-integrated optical device, which may be an optical switch, anisolator, an attenuator, a beam splitter or a beam combiner, forexample, with two collimators 26 and 28 serving as two optical ports ofthe device. An optical device element 30 may be movably positionedbetween the collimators 26 and 28. The optical device element 30 may bea prism or mirror if the optical device is an optical switch, or anattenuator or isolator element if the optical device is an isolator orattenuator, for example. The optical device typically has a metalpackaging 32 for enclosing the optical element 30. In FIG. 2, thecollimators 26 and 28 are fixed to sidewalls 34 and 36 of the metalpackaging 32, respectively. The collimators may be fixed to the sidewalls of the packaging in various conventional manners, for example, byusing epoxy gluing, tin soldering or laser welding techniques.

In a typical non-integrated optical device, such as a multi-port opticalswitch, the collimators 26 and 28 may be placed far from each other,with a distance measured in centimeters. The distances between differentcollimators in a multi-port optical device make optical alignmentbetween the collimators even more difficult. A light beam travelling infree space within an optical device typically has a very narrow beamwidth that produces a very small light spot with a Gaussiandistribution, with negligibly low light levels outside the spot area. Areceiving collimator may not collect enough optical energy even if it isslightly out of alignment with the optical path of the light beamemanating from a transmitting collimator, thereby resulting in a hugeloss of optical signals.

Alignment of collimators may be achieved in a typical non-integratedoptical device by trial and error, although the labor intensiveness ofsuch an approach is self-evident. The problem of alignment using thetrial-and-error approach is exacerbated further in a multi-port opticaldevice such as an M×N optical switch, which requires precise alignmentof many different combinations of optical paths between the collimatorsthrough different combinations of optical switching elements, such astilted mirrors or prisms. The problem associated with optical alignmentis a major factor for the high cost and slow production rate of typicalnon-integrated multi-port optical switches at the present time.

Furthermore, when the collimators are fixed to the packaging of atypical optical device, whether by using epoxy glue, tin solder or laserwelding, an assembly technician may need to continually adjust theorientation of each of the collimators while gluing, soldering orwelding the collimator to the optical device packaging. Processvariations in conventional gluing or soldering techniques may alsoultimately affect the optical alignment of the collimators. For example,epoxy glue typically takes several hours to cure, and during the curingprocess, the epoxy glue may deform slightly to cause misorientation ofthe collimators after the application of the epoxy glue. If thecollimators are soldered to the packaging of an optical device using tinsolder, for example, the alignment of the collimators may be adverselyaffected by the heating, cooling and solidifying of solders around themetal sleeves of the collimators.

In order to facilitate the alignment of collimators in an optical deviceto compensate for the effects of translational offsets and angulardeviations of light paths, optimization techniques have been proposedfor collimator alignment using a laser source, a photodetector andoptimization software on a computer. For example, a laser source (notshown) may be connected to an input optical fiber 38 which is connectedto the first collimator 26 in FIG. 2, while a photodetector (not shown)may be connected to an output optical fiber 40 which is connected to thesecond collimator 28.

The laser source provides a light beam which emanates from thecollimator 26 into the free space along an optical path 42, which has atranslational offset and an angular deviation with respect to the centeraxis 43 of the first collimator 26. The first collimator 26 may beinitially fixed to the sidewall 34 of the optical device packaging 32,while the second collimator 28 is initially movable such that it canreach the optical path 42 along which the light beam emanating from thefirst collimator 26 travels inside the optical device, to allow thephotodetector which is connected to the second collimator 28 to detectthe light beam. If the second collimator 28 can receive some opticalenergy from the light beam 42, the computer running the optimizationsoftware may at least theoretically be able to find an optimal positionand orientation for the second collimator 28 to receive the light beam42 emanating from the first collimator 26.

In practice, however, one needs to be lucky to find an initial positionand orientation for the second collimator 28 to allow it to receive atleast a detectable amount of optical energy in the first place, becausethe collimated light beam travelling along the optical path 42 mayproduce only a small light spot, for example, with a diameter as smallas 200 μm. The light spot produced by the light beam along the opticalpath 42 typically has a Gaussian distribution with very low powerdensities outside the spot area. If the photodetector that is connectedto the second collimator 28 is unable to detect the light beam in thefirst place, it would be a futile attempt for the computer optimizationsoftware to find the optimal position and orientation for the secondcollimator 28 to receive the light beam. Therefore, even with the aid ofcomputer optimization software for the alignment of optical pathsbetween different collimators in an optical device, one still needs toadjust the locations and orientations of the collimators by trial anderror to obtain at least a coarse alignment before the computer canestablish initial data points to run the optimization software to findappropriate alignment solutions.

The labor intensiveness and low productivity resulting from conventionaloptical alignment techniques are usually major contributing factors forhigh costs of manufacturing non-integrated optical devices. Furthermore,in multi-port optical devices, such as M×N optical switches, precisealignment of optical paths need be achieved for every switchablecombination of every pair of input and output collimators. With manualadjustments of collimators to obtain precise optical alignment, adequatespacings need be provided between the collimators and optical elements,such as prisms or mirrors in case of an optical switch, to allow forsuch adjustments of the collimators. The need for manual adjustments ofcollimators using conventional optical alignment techniques makes itdifficult to produce a compact non-integrated optical device with closespacings between collimators and optical elements.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for making a collimatorhaving a lens centered about a collimator axis and enclosed by an outersleeve for precise alignment of an optical path emanating from thecollimator, to obviate the need for adjusting the collimator tocompensate for a translational offset and an angular deviation of theoptical path with respect to the collimator axis. In an embodiment, theapparatus generally comprises:

a light source connected to the collimator to provide a light spot alongthe optical path;

at least one light sensor positioned to detect the light spot at a firstlongitudinal position and at a second longitudinal position;

a processor connected to said at least one light sensor to record afirst center position of the light spot at the first longitudinalposition on a transverse planar coordinate and a second center positionof the light spot at the second longitudinal position on the transverseplanar coordinate, the processor capable of determining thetranslational offset and the angular deviation of the optical path basedupon the first and second longitudinal positions of said at least onelight sensor and the first and second center positions of the light spoton the transverse planar coordinate;

a cutter rotatable about a stable axis; and

an adjustable support for holding the collimator and moving thecollimator translationally and angularly to align the optical path withthe stable axis based upon the translational offset and the angulardeviation determined by the processor, to allow the cutter to remove aportion of the outer sleeve to form an outer cylindrical surface that isconcentric with the optical path.

Advantageously, the collimator manufactured by the apparatus accordingto embodiments of the present invention with an outer cylindricalsurface that is concentric with the optical path on which the outputlight beam is transmitted from the collimator can be readily fitted toan optical device without need for further adjustment of the position ororientation of the collimator in the optical device to achieve preciseoptical alignment. Furthermore, the time and labor cost required formanufacturing various types of optical devices can be greatly reducedwith the implementation of collimators manufactured according toembodiments of the present invention, thereby significantly increasingthe productivity in optical device manufacturing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with respect to particularembodiments thereof, and references will be made to the drawings inwhich:

FIG. 1, described above, is a side sectional view of a conventionalcollimator with a GRIN lens, to illustrate a translational offset and anangular deviation of the optical path with respect to the collimatoraxis;

FIG. 2, described above, is a simplified side sectional view of atypical optical device, to illustrate the problems generally associatedwith conventional techniques of adjusting the collimators to obtainprecise optical alignment in the optical device;

FIG. 3 is a side sectional view of a collimator with a GRIN lens,illustrating the removal of a portion of the outer sleeve to form anouter cylindrical surface which is concentric with the optical pathemanating from the collimator according to an embodiment of the presentinvention;

FIG. 4 is a side sectional view of a collimator similar to FIG. 3, witha C lens instead of a GRIN lens;

FIG. 5 is a side sectional view of a collimator similar to FIG. 3, witha ball lens instead of a GRIN lens;

FIG. 6 is a side view of an embodiment of an apparatus with an airbearing and a diamond knife for machining the collimators of FIGS. 3-5according to embodiments of the present invention;

FIG. 7 shows an embodiment of an apparatus including the machiningapparatus of FIG. 6, a charge coupled device (CCD) camera and a computerfor determining the offset and deviation of the light beam emanatingfrom the collimator and aligning the collimator prior to machiningaccording to an embodiment of the present invention;

FIGS. 8A-8B illustrate images of light spots detected by the CCD camerain FIG. 7 on a transverse planar coordinate at two differentlongitudinal locations;

FIG. 9 shows center positions (x₁, y₁) and (x₂, y₂) of the light spotsin FIGS. 8A and 8B superimposed on the same planar coordinate;

FIG. 10 illustrates the relationship of the angular deviation α withrespect to the planar center positions (x₁, y₁) and (x₂, y₂) of thelight spots and a longitudinal distance Z moved by the CCD camera;

FIG. 11 shows the machining apparatus of FIG. 6 removing a portion ofthe outer sleeve of the collimator which has been moved to a tiltedposition to account for the offset and the deviation;

FIG. 12 shows the machining apparatus of FIG. 11 removing anotherportion of the outer sleeve of the collimator which has been moved tothe tilted position to account for the offset and the deviation;

FIG. 13 is a side sectional view of a finished collimator product havinga GRIN lens, with a portion of the outer sleeve removed to form an outercylindrical surface according to an embodiment of the present invention;

FIG. 14 is a side sectional view of a finished collimator productsimilar to FIG. 13, with a C lens instead of a GRIN lens;

FIG. 15 is a side sectional view of a finished collimator productsimilar to FIG. 13, with a ball lens instead of a GRIN lens;

FIG. 16 shows another embodiment of the apparatus for determining theoffset and the deviation of the light beam emanating from the collimatorwith three prisms and two charged coupled device (CCD) cameras; and

FIG. 17 is a side sectional view of a finished collimator assembly withan additional optical element, such as a filter or an isolator, at theoutput of the lens of the collimator.

DETAILED DESCRIPTION

FIG. 3 shows a side sectional view of a collimator comprising a gradedindex (GRIN) collimator lens 50, a capillary 52 positioned adjacent thelens 50 to transmit light to the lens, an optical fiber section 54connected to the capillary 52 to transmit light to the capillary 52, andan outer metal sleeve 56 enclosing the collimator lens 50 and thecapillary 52. The GRIN lens 50 and the capillary 52 have cylindricalsidewalls 58 and 60, respectively, both of which are centered about ageometric axis 62. The metal sleeve 56 has a cylindrical inner surface64 that is concentric with the GRIN lens 50 and the capillary 52, and istherefore also centered about the collimator axis 62.

Depending on the particular construction of a collimator, the innersurface 64 of the metal sleeve 56 may be in direct contact with thesidewalls of 58 and 60 the GRIN lens 50 and the capillary 52,respectively. Alternatively, a concentric layer of glass (not shown) maybe provided between the sidewalls 58 and 60 of the GRIN lens 50 and thecapillary 52 and the inner surface 64 of the metal sleeve 56 in a mannerapparent to a person skilled in the art. Whether an intermediaryconcentric layer is provided between the inner surface of the metalsleeve and the sidewalls of the collimator lens and the capillary is notcritical to the present invention.

As shown in FIG. 3, the capillary 52 has an end surface 66 which isslanted at an angle off the normal to the collimator axis 62, to preventtotal reflection of incoming light received from the optical fibersection 54 by the end surface 66 back to the optical fiber section 54.Similarly, the GRIN lens 50 also has an end surface 68 opposite the endsurface 66 of the capillary 52 and slanted at an angle off the normal tothe collimator axis 62. A gap 70 is formed between the end surfaces 66and 68 of the capillary 52 and the GRIN lens 50, respectively.

The output light beam transmitted from the collimator lens 50 into freespace usually deviates from the collimator axis 62 even if the incominglight beam entering the capillary 52 from the optical fiber section 54is initially aligned with the collimator axis 62, because of thedifferent indices of refraction n₁, n₂ and n₃ of the capillary 52, theair or gas in the gap 70, and the GRIN lens 50, respectively. Thedifferences in the indices of refraction as well as the slanted surfaces66 and 68 of the capillary 52 and the GRIN lens 50 cause the outputlight beam emanating from the GRIN lens 50 to travel along an opticalpath 72 which has an angular deviation α with respect to the collimatoraxis 62.

Furthermore, the end surfaces 66 and 68 of the capillary 52 and the GRINlens 50 may not be perfectly parallel to each other due to processvariations in the manufacturing of the collimator. The angular deviationα may be somewhat random and unpredictable because of process variationsin the manufacturing of the collimator as well as various other factors.Moreover, the incoming light beam entering the capillary 52 from theoptical fiber section 54 may not be perfectly centered about thecollimator axis 62, thereby contributing to a translational offset Δ ofthe output light beam emanating from the collimator in addition to theangular deviation α. Even within the same batch of collimators of anidentical model produced by the same manufacturer, the translationaloffset and the angular deviation may be different for each of thecollimators within the batch.

In an embodiment according to the present invention, the metal sleeve 56of the collimator is machined to produce an outer cylindrical surfacewhich is centered about an axis that coincides with the output opticalpath 72, on which the output light beam is transmitted from the GRINlens 50 of the collimator into free space with a translational offset Δand an angular deviation α with respect to the collimator axis 62. Themetal sleeve 56 typically has a cylindrical outer surface 74 that isgenerally concentric with the inner surface 64 and centered about thecollimator axis 62 before it is machined according to an embodiment ofthe present invention, although the outer surface 74 may not beperfectly cylindrical due to process variations in the initialmanufacturing of the collimator. Whether or not the outer surface 74 wasinitially cylindrical before the machining process is not critical tothe present invention.

In an embodiment according to the present invention, an outer portionindicated by sections 76, 78 of the metal sleeve 56 is removed to forman outer cylindrical surface along dashed lines 80 and 82 in the sidesectional view of FIG. 3. The outer cylindrical surface indicated bydashed lines 80 and 82 is concentric with the output light beamemanating from the GRIN lens 50 of the collimator. In other words, theouter cylindrical surface indicated by dashed lines 80 and 82 iscentered about an axis that coincides with the optical path 72 alongwhich the center of the output light beam transmitted from the GRIN lens50 travels in free space. Because the output optical path 72 has atranslational offset Δ and an angular deviation α with respect to thecollimator axis 62, the axis about which the outer cylindrical surfaceindicated by dashed lines 80 and 82 is centered is also displaced fromthe collimator axis 62 according to the translational offset Δ and theangular deviation α.

In the side sectional view shown in FIG. 3, an outer portion indicatedby sections 76, 78 the metal sleeve 56 is removed to form an outercylindrical surface indicated by dashed lines 80, 82, with atranslational offset Δ and an angular deviation α with respect to dashedreference lines 84, 86, respectively. The dashed reference lines 84 and86 may be initially chosen arbitrarily and set at an equal distance fromthe collimator axis 62, provided that these reference lines are chosento allow sufficient removal of the outer portion 76, 78 of the metalsleeve 56 to form the outer cylindrical surface that is concentric withthe output optical path 72, while avoiding excessive removal of themetal sleeve.

FIG. 4 shows a side sectional view of a collimator similar to FIG. 3,with the GRIN lens 50 in FIG. 3 replaced by a C lens 88 with a curvedcollimating surface 90. FIG. 5 shows a side sectional view of acollimator similar to FIG. 3, with a hemispheric ball lens 92 replacingthe GRIN lens 50 in FIG. 3. The outer metal sleeves 56 of these types ofcollimators may be machined to form outer cylindrical surfaces tocompensate for translational offsets and angular deviations of outputoptical paths in a similar manner to that which is described above withreference to FIG. 3.

Collimators with other types of lenses or other optical structures canalso be machined to compensate for possible translational offsets andangular deviations in a similar manner. After the outer sleeve of eachcollimator is machined to form an outer cylindrical surface which iscentered about an axis that coincides with the output optical pathemanating from the collimator, the collimator is ready for fitting to anoptical device to achieve precise alignment of optical paths, whileobviating the need for further adjustment of the position or orientationof each collimator to compensate for the translational offset or angulardeviation as would be required in a time-consuming, labor-intensiveconventional alignment process.

For a typical collimator, the angular deviation and the translationaloffset are usually very small. In FIGS. 3-5 described above, the angulardeviation α as well as the translational offset Δ may be exaggerated forthe purpose of illustration. In reality, however, even a small angulardeviation and a small translational offset may cause great difficultiesin a conventional alignment process. Therefore, the removal of the outerportion 76, 78 of the metal sleeve 56 in FIGS. 3-5 need be achieved veryprecisely. Furthermore, the angular deviation α and the translationaloffset Δ of the output optical path 72 with respect to the collimatoraxis 62 need be measured very precisely instead of by trial and errorbefore the metal sleeve 56 is machined to form the outer cylindricalsurface to compensate for the translational offset and the angulardeviation.

FIG. 6 shows an embodiment of an apparatus for machining the outer metalsleeve of a collimator to form an outer cylindrical surface tocompensate for a translational offset and an angular deviation. In thisembodiment, an air bearing 100 is provided which is capable of rotatingabout a stable bearing axis 102. An air bearing that is capable of avery high rate of rotation, for example, approximately 10,000 rotationsper minute (rpm), with a very high degree of axial stability iscommercially available at the present time. In a typical air bearing, aninner cylinder 104 is surrounded by an outer annulus 106 without directcontact. The inner cylinder 104 is separated from the outer annulus 106by a gap 108 filled with pressurized air or gas. The pressurized air maybe supplied by a high pressure air supply 110, such as an air pump or apressurized air bottle.

In the embodiment shown in FIG. 6, the outer annulus 106 of the airbearing 100 is fixed while the inner cylinder 104 is rotating. In anembodiment, an electric motor 112, such as a stepper motor, drives theinner cylinder 104 at a high rate of rotation. In the embodiment shownin FIG. 6, the inner cylinder 104 of the air bearing 100 is connected toa wheel 114, which is driven by a belt 116 connected to the electricmotor 112. The driving mechanism for the air bearing 100 is not criticalto the present invention, as long as the air bearing is capable ofmaintaining a very high degree of axial stability while rotating.

In the embodiment shown in FIG. 6, a cutter 118 is provided on the innercylinder 104 of the air bearing 100, to cut a portion of the metalsleeve of a collimator to form the desired outer cylindrical surfacewhich is concentric with the output optical path emanating from thecollimator. Because the metal sleeve of a conventional collimator istypically made of a hard material such as stainless steel, it isdesirable that the cutter 118 be capable of cutting the metal sleevewith a high degree of stability and precision. In an embodiment, thecutter 118 comprises a diamond knife with a precise straight cuttingedge for cutting the outer portion of the metal sleeve. The diamondcutting knife 118, which is attached to the air bearing 100, is capableof cutting the metal sleeve portion by rotating about the bearing axis102 with a very high degree of stability, because the inner cylinder 104of the air bearing 100 is capable of rotating at a high rate of rotationabout the bearing axis 102 with a very high degree of concentricity.

FIG. 7 shows an apparatus for determining the translational offset andthe angular deviation of the output optical path of a typical collimatorbefore the metal sleeve of the collimator is machined by the apparatusof FIG. 6. In FIG. 7, the air bearing 100 to which the diamond cuttingknife 118 is attached provides a highly stable bearing axis 102 that isused as a reference for the determination of the translational offsetand the angular deviation.

The collimator 120 is held securely on an adjustable support 122 whichcan be moved translationally and angularly. The adjustable support 122is capable of moving the collimator 120 very precisely with fineadjustments in translational or angular displacement, either manually orby an automatic controller. The collimator 120 is connected to a lightsource 124, such as a laser source capable of emitting a laser beam at adesired wavelength, through an optical fiber 126, such as a single modefiber. The light source 124 transmits a light beam through thecollimator 120 to provide a light spot along an output optical path 128emanating from the collimator 120. Depending upon the wavelengthspectrum used by the optical fiber system in which the collimator is tobe implemented, the light source 124 may be a laser source that emits alaser beam at a center wavelength of that spectrum. For example, atypical optical fiber communications network may utilize infraredspectra around the wavelengths of 1300 nm and 1550 nm.

In an embodiment, a light sensor such as a charge coupled device (CCD)camera 130 is provided to detect the light spot generated by the lightsource 124 through the collimator 120 at two different longitudinalpositions z₁ and z₂ along the bearing axis 102. In the embodiment shownin FIG. 7, the CCD camera 130 is positioned on a sliding arm 132 whichis movable longitudinally along a guide rail 134. In this embodiment,the guide rail 134 is parallel to the axis 102 of the air bearing 100 toallow the CCD camera 130 to move only in the longitudinal directionswhile detecting images of the light spot.

In an embodiment, the images of the light spot produced by thecollimator 120 and detected by the CCD camera 130 are captured by acomputer processor 136 and shown on a display 138, such as a computermonitor. The computer processor 136 may be connected to the CCD camera130 by a conventional interface. The CCD camera 130 is initially locatedat a first predetermined longitudinal position z₁, to allow the computerprocessor 136 to capture a first image of the light spot on a transverseplanar coordinate, for example, a Cartesian coordinate (x, y) as shownin FIG. 8A. The image of the light spot 140, which typically has asubstantially circular or elliptical shape as shown in FIG. 8A, isdisplayed on the computer monitor 138 in FIG. 7 with reference to aCartesian coordinate with an x-axis and a y-axis. Based upon the imageof the light spot 140 as shown in FIG. 8A, the computer determines afirst center point (x₁, y₁) of the light spot 140 when the CCD camera isat the first longitudinal position z₁ as shown in FIG. 7.

After the CCD camera 130 has moved longitudinally from the firstlongitudinal position z₁ to the second longitudinal position z₂ by adistance Z as shown in FIG. 7, a second image of the light spot 142 asshown in FIG. 8B is captured by the computer processor 136 and displayedon the monitor 138. Based upon this image, the computer determines asecond center point (x₂, y₂) of the light spot 142 as shown in FIG. 8Bon the same transverse planar Cartesian coordinate as the one shown inFIG. 8A. If the output light path 128 emanating from the collimator 120as shown in FIG. 7 has an angular deviation with respect to the axis 102of the air bearing 100, the second center position (x₂, y₂) of the lightspot 142 shown in FIG. 8B when the CCD camera 130 is at the secondlongitudinal position z₂ would be different from the first centerposition (x₁, y₁) of the light spot 140 shown in FIG. 8A when the CCDcamera 130 is at the first longitudinal position z₁.

In order to compensate for a possible translational offset of the outputlight beam emanating from the collimator in addition to the angulardeviation, the origin of the Cartesian coordinate, which serves as areference for the center points of the light spots as shown in FIGS. 8Aand 8B, may be set at a predetermined point along the axis 102 of theair bearing 100 as shown in FIG. 7. In an embodiment, an arbitrary pointon the bearing axis 102 is selected as an absolute reference point asthe first longitudinal position z₁ for the CCD camera 130. The initialtransverse position of the CCD camera 130 at the first longitudinalposition z₁ is adjusted until the center point of the CCD camera matchesthe point which is selected as the absolute reference point on thebearing axis 102. After the CCD camera 130 has been centered at thatpoint along the bearing axis 102, the transverse position of the CCDcamera is fixed on the sliding arm 132 and subsequently can only bemoved longitudinally parallel to the guide rail 134.

When the CCD camera 130 is moved from the first longitudinal position z₁to the second longitudinal position z₂, the origin of the transverseplanar Cartesian coordinate does not change with respect to the bearingaxis 102. If an angular deviation exists in the light beam emanatingfrom the collimator 120, the center position of the light spot on thetransverse Cartesian coordinate when the CCD camera is at the secondlongitudinal position z₂ would be different from the center position ofthe light spot on the same coordinate when the CCD camera is at thefirst longitudinal position z₁. After the center points of the lightspots at the two longitudinal positions of the CCD camera are obtainedby the computer, the computer may superimpose these two center points onthe Cartesian coordinate on the computer monitor as shown in FIG. 9. Thetransverse distance D between these two center points is {square rootover ((x₂−x₁)²+(y₂−y₁)²)}.

FIG. 10 illustrates the relationship of the angular deviation α of theoutput light beam with respect to the transverse distance D and thelongitudinal distance Z on a three-dimensional Cartesian coordinate (x,y, z). In FIG. 10, the first and second center points (x₁, y₁) and (x₂,y₂) are identical to those shown in FIG. 9. The vertical displacement zis the distance between the first and second longitudinal positions z₁and z₂ of the CCD camera 130 as shown in FIG. 7. Referring to FIG. 10,the angular deviation α can be derived from the transverse distance Dbetween the first and second center points of the light spot and thelongitudinal distance Z according to the following relationship:α=arctan D/Z. This angular deviation may be computed by the processor136 automatically. Alternatively, the images of the light spots as shownin FIGS. 8A and 8B obtained by the CCD camera 130 at respective firstand second longitudinal positions z₁ and z₂ may be captured on thedisplay 138 for a human operator to calculate the angular deviation.

After the center positions of the light spot as shown in FIGS. 8A and 8Bare obtained and the angular deviation α determined for the outputoptical path of the collimator 120 in FIG. 7, the support 122 thatsecurely holds the collimator 120 is adjusted until the light spotdetected by the CCD camera 130 is centered at the origin of thetransverse Cartesian coordinate (x, y). The adjustable support 122 maybe moved manually with fine tuning, or automatically controlled by thecomputer processor 136, which has already determined the angulardeviation α.

If only a translational offset but no angular deviation is present forthe output light beam emanating from the collimator 120 in FIG. 7, theadjustable support 122 need only be moved transversely until the lightspot detected by the CCD camera is centered at the origin of thetransverse Cartesian coordinate (x, y), thereby aligning the center axisof the output optical path 128 with the axis 102 of the air bearing 100.If only an angular deviation but no translational offset is present forthe output light beam emanating from the collimator 120, the adjustablesupport 122 that securely holds the collimator 120 need be rotatedangularly until the light spot detected by the CCD camera 130 iscentered at the origin of the Cartesian coordinate (x, y).

If both a translational offset and an angular deviation are present forthe light beam emanating from the collimator 120, the adjustable support122 may need be moved both transversely and angularly until the lightspot detected by the CCD camera 130 is centered at the origin of thetransverse Cartesian coordinate (x, y). In any of these scenarios, theoutput optical path along which the light beam emanating from thecollimator 120 travels in free space is aligned with the axis 102 of theair bearing 100. The alignment is performed when the center of the CCDcamera 130 is on the bearing axis 102, regardless of the longitudinalposition of the CCD camera. During the processes of detecting andcapturing of the light spot, determining the angular deviation, andmoving the adjustable support to achieve optical alignment with thebearing axis, the inner cylinder 104 of the air bearing 100 maintains ahigh rate of rotation such that the bearing axis 102 is highly stablefor providing an absolute reference as the origin of the transverseplanar coordinate (x, y).

After the position and orientation of the collimator 120 have beenadjusted for precise alignment of the output optical path with the axis102 of the air bearing 100, the CCD camera is removed from the outputoptical path of the collimator. The adjustable support 122 that securelyholds the collimator 120 is moved longitudinally toward the cutter 118which is attached to the rotating air bearing 100, to remove a portionof the metal sleeve of the collimator to compensate for thetranslational offset Δ and the angular deviation α of the output opticalpath according to FIG. 3. To ensure precise removal of the outer portionof the metal sleeve without deformation, the cutter 118 comprises adiamond knife with a straight cutting edge 119 in an embodiment toproduce the outer cylindrical surface which is concentric with theoutput light beam.

FIG. 11 shows the machining apparatus with a diamond cutting knife 118attached to the rotating inner cylinder 104 of the air bearing 100 as itremoves a portion indicated by section 146 of the metal sleeve 144 ofthe collimator 120. Similarly, FIG. 12 illustrates the removal of aportion indicated by section 148 of the metal sleeve 144 when thediamond knife 118 is rotated to another side of the collimator. Theportion 146, 148 of the metal sleeve 144 is removed to form the outercylindrical surface indicated by lines 150 and 152, respectively, in theside sectional view of FIG. 13.

In FIG. 13, the outer cylindrical surface 150, 152 formed by the diamondcutting knife on the metal sleeve 144 of the collimator 120 isconcentric with the optical path 154 along which the output light beamemanating from the collimator travels in free space with a translationaloffset Δ and an angular deviation α with respect to the collimator axis62. In other words, the outer cylindrical surface 150, 152 of the metalsleeve 144 is centered about an axis which is displaced from thecollimator axis 62 according to the translational offset Δ and theangular deviation α.

In FIG. 11, the location of the cutting edge 119 of the diamond knife118 with respect to the bearing axis 102 is determined according to thedashed reference lines 84 and 86 as shown in FIG. 3, to avoid excessivecutting of the metal sleeve while ensuring that a sufficiently largeouter cylindrical surface is formed for ready fitting to an opticaldevice. The machined surfaces 150 and 152 of the outer sleeve 144 of thecollimator as shown in FIG. 13 are ready for fitting to an opticaldevice using a conventional technique, for example, laser welding, suchthat the output light beam is transmitted from the collimator lens alongthe optical path 154 coinciding with an axis about which the machinedouter cylindrical surfaces 150 and 152 are centered.

The side sectional view of FIG. 13 shows a finished collimator productwith a GRIN lens 160 and a capillary 162 enclosed by the metal sleeve144, which has an outer cylindrical surface 150, 152 that is concentricwith the light beam emanating from the GRIN lens 160. FIG. 14 shows aside sectional view similar to FIG. 13, with a C lens 164 which has acurved outer surface 166 instead of the GRIN lens. FIG. 15 shows a sidesectional view of a collimator similar to FIG. 13, with a hemisphericball lens 168 instead of the GRIN lens. The apparatus according toembodiments of the present invention can also be used for machining manyother types of collimators to compensate for possible offsets ordeviations of optical paths regardless of the particular type orconstruction of optical elements within the collimators.

FIG. 16 shows a preferred embodiment of the apparatus for determiningthe offset and deviation of the light beam emanating from thecollimator, with two fixed CCD cameras for detecting the light spotsinstead of a single moving CCD camera as shown in FIG. 7 and describedabove. In the embodiment shown in FIG. 16, a focusing lens 202 ispositioned along the output optical path 204 emanating from thecollimator 120. In this embodiment, the rotating cylinder 104 of the airbearing 100 has a cylindrical hollow interior 206 to allow the lighttraveling along the output optical path 204 to pass through the airbearing 100.

In this embodiment, a total-reflection prism 208 is positioned toreflect the light beam that has traveled through the hollow interior 206of the rotating cylinder 104 of the air bearing 100 onto a reflectedoptical path 210. As shown in FIG. 16, two additional prisms including ahalf-reflection prism 212 and a total-reflection prism 214 arepositioned along the optical path 210 to reflect the output light beamonto separate optical paths 216 and 218, respectively. The prisms 208,212 and 214 are fixed in position with respect to each other and withrespect to the air bearing 100.

Two CCD cameras 220 and 222 are positioned to detect the light beamstraveling along the reflected output optical paths 216 and 218,respectively. The CCD cameras 220 and 222 are fixed in space withrespect to each other and with respect to the half-reflection prism 212and the total-reflection prism 214. In this embodiment, the CCD cameras220 and 222 are connected to a computer monitor 224 which displays thelight spots sensed by the CCD cameras 220 and 222 on a Cartesiancoordinate (x, y).

If the output optical path 204 emanating from the collimator 120 isperfectly aligned with the center axis 102 of the air bearing 100, thelight spots detected by the two CCD cameras 220 and 222 would both becentered about the origin of the Cartesian coordinate. If the outputoptical path 204 has a translational offset with respect to the centeraxis 102 of the air bearing 100, the centers of the light spots detectedby the CCD cameras 220 and 222 would be offset from the origin of theCartesian coordinate. If an angular deviation exists in the outputoptical path 204 with respect to the center axis 102, the monitor 224would show two separate light spots detected by the CCD cameras 220 and222.

In FIG. 16, the computer monitor 224 shows a first light spot 226 with afirst center 228 and a second light spot 230 with a second center 232detected by the CCD cameras 220 and 222, indicating that both atranslational offset and an angular deviation exist in the outputoptical path 204 with respect to the axis 102 of the air bearing 100.The positions of the centers 228 and 232 of the light spots 226 and 230on the Cartesian coordinate (x, y) allow a computer to determine theappropriate adjustments that need be made to the position andorientation of the collimator 120 to align the output optical path 204emanating from the collimator 120 with the center axis 102 of the airbearing 100.

In an embodiment, the support plate 122 that securely holds thecollimator 120 can moved translationally and angularly with fineadjustments to align the optical path 204 with the axis 102 of the airbearing 100. Once the optical path 204 is aligned with the center axis102 of the air bearing 100, the light spots detected by the CCD cameras220 and 222 would overlap each other and would be centered about theorigin of the Cartesian coordinate on the monitor 224.

The apparatus according to embodiments of the present invention is notlimited to the machining of collimators themselves. It is typical thatan extra optical element may be added to the output of a collimator.Such an extra optical element may be an optical filter or an opticalisolator, for example. The apparatus according to embodiments of thepresent invention may also be used for machining such a collimatorassembly with an additional optical element. In an embodiment, the extraoptical element, such as a filter or an isolator, is fitted to thecollimator to form a collimator assembly before the offset and deviationof the output optical path emanating from the collimator are measured.In this embodiment, the overall offset and deviation of the collimatorassembly take into account any possible offset or deviation due to theextra optical element as well as the collimator lens. After the overalloffset and deviation are measured, the sleeve of the collimator ismachined to form the desired outer cylindrical surface.

FIG. 17 shows a collimator assembly similar to FIG. 13, but with anextra optical element 240 positioned adjacent the output of the lens 160of the collimator. This extra optical element may be a filter, anisolator, or any other type of optical device. The sleeve 144 of thecollimator assembly is machined to form the outer cylindrical surface150, 152 in a similar manner to FIG. 13.

The present invention has been described with respect to particularembodiments thereof, and numerous modifications can be made which arewithin the scope of the invention as set forth in the claims.

What is claimed is:
 1. An apparatus for making a collimator having alens centered about a collimator axis and enclosed by an outer sleeve,for precise alignment of an optical path emanating from the collimatorto obviate need for adjusting the collimator to compensate for atranslational offset or an angular deviation of the optical path withrespect to the collimator axis, the apparatus comprising: a light sourceconnected to the collimator to provide a light spot along the opticalpath; at least one light sensor positioned to detect the light spot at afirst longitudinal position and at a second longitudinal position; aprocessor connected to said at least one light sensor to record a firstcenter position of the light spot at the first longitudinal position ona transverse planar coordinate and a second center position of the lightspot at the second longitudinal position on the transverse planarcoordinate, the processor capable of determining the translationaloffset and the angular deviation of the optical path based upon thefirst and second longitudinal positions of said at least one lightsensor and the first and second center positions of the light spot onthe transverse planar coordinate; a cutter rotatable about a stableaxis; and an adjustable support for holding the collimator and movingthe collimator translationally and angularly to align the optical pathwith the stable axis based upon the translational offset and the angulardeviation determined by the processor, to allow the cutter to remove aportion of the outer sleeve to form an outer cylindrical surface that isconcentric with the optical path.
 2. The apparatus of claim 1, whereinthe cutter comprises a bearing capable of rotating about the stableaxis.
 3. The apparatus of claim 2, wherein the bearing comprises an airbearing.
 4. The apparatus of claim 3, wherein the air bearing comprisesan inner cylinder and an outer annulus separated by a gap filled withpressurized air.
 5. The apparatus of claim 3, wherein the cutter furthercomprises a diamond knife connected to the air bearing.
 6. The apparatusof claim 1, wherein said at least one light sensor comprises a chargecoupled device (CCD) camera.
 7. The apparatus of claim 1, wherein saidat least one light sensor comprises a light sensor that is movablelongitudinally to detect the light spot at the first and secondlongitudinal positions.
 8. The apparatus of claim 7, further comprisinga guide rail connected to the light sensor to allow the light sensor tomove to the first and second longitudinal positions.
 9. The apparatus ofclaim 1, wherein the processor comprises a computer.
 10. The apparatusof claim 1, wherein the light source comprises a laser source.
 11. Theapparatus of claim 1, wherein the lens comprises a graded index (GRIN)lens.
 12. The apparatus of claim 1, wherein the lens comprises a C lens.13. The apparatus of claim 1, wherein the lens comprises a ball lens.14. The apparatus of claim 1, wherein the collimator further comprises acapillary adjacent the lens to transmit light to the lens.
 15. Theapparatus of claim 14, further comprising a fiber connected between thecapillary and the light source.
 16. The apparatus of claim 1, whereinthe sleeve comprises a metal sleeve.
 17. The apparatus of claim 1,wherein the transverse planar coordinate is a Cartesian coordinatehaving an x-axis and a y-axis intersecting at an origin, wherein theorigin is set at a given point along the stable axis, and wherein thefirst center position is at (x1, y1) and the second center position isat (x2, y2).
 18. The apparatus of claim 17, wherein the processorcomputes a distance D between the first center position and the secondcenter position on the Cartesian coordinate according to a relationship:D={square root over ((x₂−x₁)²+(y₂−y₁)²)}.
 19. The apparatus of claim 18,wherein the second longitudinal position is separated from the firstlongitudinal position by a longitudinal distance Z.
 20. The apparatus ofclaim 19, wherein the processor computes the deviation α according to arelationship: α=arctan D/Z.
 21. The apparatus of claim 1, furthercomprising a display connected to the processor to display the lightspot on the transverse planar coordinate.
 22. An apparatus for making acollimator having a lens centered about a collimator axis and enclosedby an outer sleeve, for precise alignment of an optical path emanatingfrom the collimator to obviate need for adjusting the collimator tocompensate for a translational offset or an angular deviation of theoptical path with respect to the collimator axis when the collimator isassembled to an optical device, the apparatus comprising: a light sourceconnected to the collimator to provide a light spot along the opticalpath; a light sensor movably positioned to detect the light spot at afirst longitudinal position and at a second longitudinal position; aprocessor connected to the light sensor to record a first centerposition of the light spot at the first longitudinal position on atransverse planar coordinate and a second center position of the lightspot at the second longitudinal position on the transverse planarcoordinate, the processor capable of determining the translationaloffset and the angular deviation of the optical path based upon thefirst and second longitudinal positions of the light sensor and thefirst and second center positions of the light spot on the transverseplanar coordinate; a bearing capable of rotating about a stable bearingaxis; a cutter connected to the bearing; and an adjustable support forholding the collimator and moving the collimator translationally andangularly to align the optical path with the stable axis based upon thetranslational offset and the angular deviation determined by theprocessor, to allow the cutter to remove a portion of the outer sleeveto form an outer cylindrical surface that is concentric with the opticalpath.
 23. The apparatus of claim 22, wherein the bearing comprises anair bearing.
 24. The apparatus of claim 23, wherein the air bearingcomprises an inner cylinder and an outer annulus separated by a gapfilled with pressurized air.
 25. The apparatus of claim 23, wherein thecutter comprises a diamond knife connected to the air bearing.
 26. Theapparatus of claim 22, wherein the light sensor comprises a chargecoupled device (CCD) camera.
 27. The apparatus of claim 26, furthercomprising a guide rail connected to the CCD camera to allow the CCDcamera to move to the first and second longitudinal positions.
 28. Theapparatus of claim 22, wherein the processor comprises a computer. 29.The apparatus of claim 22, wherein the light source comprises a lasersource.
 30. The apparatus of claim 22, wherein the lens comprises agraded index (GRIN) lens.
 31. The apparatus of claim 22, wherein thelens comprises a C lens.
 32. The apparatus of claim 22, wherein the lenscomprises a ball lens.
 33. The apparatus of claim 22, wherein thecollimator further comprises a capillary adjacent the lens to transmitlight to the lens.
 34. The apparatus of claim 33, further comprising afiber connected between the capillary and the light source.
 35. Theapparatus of claim 22, wherein the sleeve comprises a metal sleeve. 36.The apparatus of claim 22, wherein the transverse planar coordinate is aCartesian coordinate having an x-axis and a y-axis intersecting at anorigin, wherein the origin is set at a given point along the stablebearing axis, and wherein the first center position is at (x1, y1) andthe second center position is at (x2, y2).
 37. The apparatus of claim36, wherein the processor computes a distance D between the first centerposition and the second center position on the Cartesian coordinateaccording to a relationship: D={square root over ((x₂−x₁)²+(y₂−y₁)²)}.38. The apparatus of claim 37, wherein the second longitudinal positionis separated from the first longitudinal position by a longitudinaldistance Z.
 39. The apparatus of claim 38, wherein the processorcomputes the deviation α according to a relationship: α=arctan D/Z. 40.The apparatus of claim 22, further comprising a display connected to theprocessor to display the light spot on the transverse planar coordinate.41. An apparatus for making a collimator having a lens centered about acollimator axis and enclosed by an outer sleeve, for precise alignmentof an optical path emanating from the collimator to obviate need foradjusting the collimator to compensate for a translational offset or anangular deviation of the optical path with respect to the collimatoraxis, the apparatus comprising: a light source connected to thecollimator to provide a light spot along the optical path; at least onelight sensor positioned to detect the light spot at a first longitudinalposition and at a second longitudinal position; a display connected tosaid at least one light sensor to display a first center position of thelight spot at the first longitudinal position on a transverse planarcoordinate and a second center position of the light spot at the secondlongitudinal position on the transverse planar coordinate; a cutterrotatable about a stable axis; and an adjustable support for holding thecollimator and moving the collimator translationally and angularly toalign the optical path with the stable axis based upon the first andsecond center positions of the light spot on the display, to allow thecutter to remove a portion of the outer sleeve to form an outercylindrical surface that is centered about an axis which coincides withthe optical path.
 42. The apparatus of claim 41, wherein the cuttercomprises a bearing capable of rotating about the stable axis.
 43. Theapparatus of claim 42, wherein the bearing comprises an air bearing. 44.The apparatus of claim 43, wherein the air bearing comprises an innercylinder and an outer annulus separated by a gap filled with pressurizedair.
 45. The apparatus of claim 43, wherein the cutter further comprisesa diamond knife connected to the air bearing.
 46. The apparatus of claim41, wherein said at least one light sensor comprises a charge coupleddevice (CCD) camera.
 47. The apparatus of claim 41, wherein said atleast one light sensor comprises a light sensor that is movablelongitudinally to detect the light spot at the first and secondlongitudinal positions.
 48. The apparatus of claim 47, furthercomprising a guide rail connected to the light sensor to allow the lightsensor to move to the first and second longitudinal positions.
 49. Theapparatus of claim 41, further comprising a processor connected to saidat least one sensor to compute the translational offset or the angulardeviation.
 50. The apparatus of claim 41, wherein the light sourcecomprises a laser source.
 51. The apparatus of claim 41, wherein thelens comprises a graded index (GRIN) lens.
 52. The apparatus of claim41, wherein the lens comprises a C lens.
 53. The apparatus of claim 41,wherein the lens comprises a ball lens.
 54. The apparatus of claim 41,wherein the collimator further comprises a capillary adjacent the lensto transmit light to the lens.
 55. The apparatus of claim 54, furthercomprising a fiber connected between the capillary and the light source.56. The apparatus of claim 41, wherein the sleeve comprises a metalsleeve.
 57. An apparatus for making a collimator having a lens centeredabout a collimator axis and enclosed by an outer sleeve, for precisealignment of an optical path emanating from the collimator to obviateneed for adjusting the collimator to compensate for a translationaloffset or an angular deviation of the optical path with respect to thecollimator axis, the apparatus comprising: a light source connected tothe collimator to produce an output light beam; at least one reflectorpositioned to reflect the output light beam onto first and secondreflected output optical paths; first and second light sensorspositioned to detect first and second light spots on the first andsecond reflected output optical paths, respectively; a display connectedto the light sensors to display the first light spot detected by thefirst light sensor and the second light spot detected by the secondlight sensor on a transverse planar coordinate; a cutter rotatable abouta stable axis; and an adjustable support for holding the collimator andmoving the collimator translationally and angularly to align the outputlight beam with the stable axis based upon positions of the first andsecond light spots on the display, to allow the cutter to remove aportion of the outer sleeve to form an outer cylindrical surface that iscentered about an axis which coincides with the optical path.
 58. Theapparatus of claim 57, wherein the cutter comprises a bearing capable ofrotating about the stable axis.
 59. The apparatus of claim 58, whereinthe bearing comprises an air bearing.
 60. The apparatus of claim 59,further comprising a lens positioned between the collimator and said atleast one reflector.
 61. The apparatus of claim 57, wherein said atleast one reflector comprises at least one total-reflection prism and atleast one half-reflection prism.
 62. The apparatus of claim 57, whereinthe first and second light sensors comprise first and second chargecoupled device (CCD) cameras, respectively.